Unlike gradual climate-driven events, this potential earthquake would “happen within minutes, leaving no time for adaptation or mitigation,” These are the words of Virginia Tech Department of Geosciences assistant professor Tina Dura to the essence of a scenario that has transitioned from hypothesis to pressing reality for the Pacific Northwest. The latest research, in the Proceedings of the National Academy of Sciences, estimates a 15 percent chance within the next 50 years of a magnitude 8.0 or greater earthquake on the Cascadia Subduction Zone a number that has focused intense interest from geoscientists, engineers, and coastal planners.
The Cascadia Subduction Zone, which stretches about 700 miles from the northern end of Vancouver Island to Cape Mendocino, California, is a plate boundary where the Juan de Fuca Plate is being pushed beneath the North American Plate. The geological setup is infamous for generating “megathrust” earthquakes events not merely strong enough to shake the ground but powerful enough to reconfigure the land in an elemental way. The last grand rupture, in 1700, deposited a geological record of abrupt coast subsidence and trans-Pacific tsunamis, Japanese records noting the appearance of an “orphan wave” several hours after the ground stopped shaking in North America.
The shaking itself is not what characterizes the Cascadia hazard, but the potential for calamitous land subsidence and creation of a 1,000-foot “mega-tsunami.” In contrast to normal tsunamis, which tend to create waves that are only a few feet high, mega-tsunamis are characterized by their world-record-breaking height and travel, occasionally reaching miles inland and consuming entire settlements (New York Post). The physics are as dramatic as the effects: when the tectonic plates move suddenly, the ground beneath the ocean rises abruptly, pushing massive amounts of water aside and creating a wall of water that plows ashore.
The physics of those waves have been extensively modeled using classical numerical methods and, more recently, machine learning. Physics-informed neural network advancements enable scientists to simulate tsunami propagation and inundation at very high and precise velocities, even accounting for the highly complex ocean depth-coastal geology interaction as well as diverse frictional forces. These models are guided by the histories of occurrences like the 1958 Lituya Bay mega-tsunami in Alaska, in which a wave triggered by a landslide was an incredible 1,719 feet high, and by the geologic histories of the ancient tsunamis that have sculpted the Pacific Northwest. The consequences for the region are staggering.
The new study places the estimate of land that may be inundated by up to 6.5 feet in a single incident at flooding the local sea level and widening the floodplain simultaneously. As Dura breaks down, “The expansion of the coastal floodplain following a Cascadia subduction zone earthquake has not been previously quantified, and the impacts to land use could significantly increase the timeline to recovery.” That expansion is substantial: the number of locations that are at risk of flooding would increase more than threefold by 2100 when factoring in climate-driven sea-level rise, bringing thousands more people, structures, and miles of roadway into the hazard zone.
For the Pacific Northwest, the worst impacts are forecast to strike southern Washington, northern Oregon, and northern California. A Cascadia-generated tsunami has much longer reach, however, with Alaska and Hawaii still at risk owing to their own seismic and volcanic records. In Hawaii, for example, failure of volcanic slopes has created ancient mega-tsunamis, and ongoing activity by volcanoes such as Mauna Loa and Kilauea also still threaten the islands.
Science related to tsunami warning has improved by leaps and bounds from the 2004 Indian Ocean tragedy. The United States, together with NOAA and research organizations, has installed a chain of Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys that offer real-time information about the wave travel. These are complemented by sophisticated forecasting models like the Short-term Inundation Forecasting for Tsunamis (SIFT) system, which has the capability to combine seismic and oceanographic information to make community-level inundation forecasts. Despite this, the problem is: in the case of near-field events such as a Cascadia rupture, the warning period might be measured in minutes rather than hours.
New research has begun to investigate how artificial intelligence and emerging sensor technology, including hydrophones able to pick up the distinctive seafloor acoustic signature of earthquakes, might be brought into play. With traditional seismic and pressure sensors, the instruments promise quicker and more precise warnings but continuing deployment and integration into functional systems.
The dangers extend beyond human life and property. The sharp reduction in intertidal wetlands, estuaries, and harboring dunes would have radiating impacts on ecosystem services ranging from water purification and fishery habitat to carbon storage. As pointed out by Dura, “The loss of intertidal wetlands directly impacts ecosystem services such as water filtration, habitat for fisheries and shorebirds, and carbon storage capacity,” Loss or erosion of these natural buffers would reduce their carbon sequestration capability and future storm surge protection.
The engineering community has reacted by creating new codes for structures, evacuation plans, and hazard maps that address seismic as well as climate danger. But as the USGS and others highlight, much risk analysis today remains yet to be completely incorporated with the two-way risk of earthquake-caused subsidence and sea-level rise.
The history of Cascadia is that of a sleeping giant its long period of time between big quakes putting neighborhoods into a state of complacency despite the fact that 11 great Cascadian earthquakes have struck in the past 6,000 to 7,000 years, 200 to 800 years apart. And as Dura goes on to say, “Cascadia is a unique place. It’s not super heavily populated, but most estuaries have a community in them, and they’re all right in the zone of subsidence.”
For the residents and experts in the Pacific Northwest, the intersection of tectonic science, sophisticated modeling, and adaptive engineering will decide whether the region is ready to anticipate an inevitable future where the next great Cascadia earthquake is not a matter of if but when.
